Security alarm system

ABSTRACT

Security alarm system of the McCulloh loop type in which the polarity of the voltage applied to one end of the loop is automatically reversed when abnormal conditions exist in the loop, thus maintaining the loop in an operable condition despite the occurrence of openings and/or shorts to ground in the loop. The voltage is automatically restored to its normal polarity upon correction of the opening and/or grounding.

United States Patent Ajmani et a1.

[ 1 May 9,1972

[54] SECURITY ALARM SYSTEM [72] Inventors: Mohan Ajmani; Larry K. De Lowe, both of Dublin; Thomas C. Hankins, Oakland,

all of Calif.

[73] Assignee: Systron-Donner Corporation, Concord,

Calif.

[22] Filed: Aug.7, 1970 211 App]. No.: 62,052

[52] U.S. Cl ..340/409, 179/5, 340/276 [51] ..G08b 19/00 [58] Field of Search ..340/409, 276, 215; 178/4.1,

[56] References Cited UNITED STATES PATENTS 3,351,934 11/1967 Vietz ..340/2l5 X 533,254 l/1895 Turner ..340/215 1,688,711 10/1928 Hewitt et a1. ..l78/4.1 3,387,087 6/1968 Kleinschmidt ..178/89 Primary Examiner-David L. Trafton Att0rney-Fleht', l-lohbach, Test, Albritton & Herbert [57] ABSTRACT Security alarm system of the McCulloh loop type in which the polarity of the voltage applied to one end of the loop is automatically reversed when abnormal conditions exist in the loop, thus maintaining the loop in an operable condition despite the occurrence of openings and/or shorts to ground in the loop. The voltage is automatically restored to its normal polarity upon correction of the opening and/or grounding.

13 Claims, 2 Drawing Figures PowE R 27 SUPPLY PATENTEUHAY 91972 3, 662.383

|2 I |2 TRANSMITTER TRANSMITTER Q '8 2| 22 F 6- |0O INVENTORS MOHAN L. AJMANI LARRY K. DELOWE THOMAS C. HANKINS ATTORNEYS SECURITY ALARM SYSTEM REFERENCE TO RELATED APPLICATION This invention relates to the invention disclosed in application, Ser. No. 50,233 filed June 26, 1970 BACKGROUND OF THE INVENTION This invention pertains generally to security alarm systems and more particularly to the type of system known as a Mo- Culloh system.

A McCulloh system comprises a protective loop which extends from the monitoring station to one or more protected stations and then back to the monitoring station. Each protected station includes a transmitter for transmitting an alarm signal through the loop. The alarm signals are commonly pulse coded to permit identification at the monitoring station of the transmitter from which they originate. The loop generally consists of a single electrically conductive line such as a leased telephone line, and one loop typically can accommodate on the order of 50 protected stations. These stations can be located in buildings, such as businesses and warehouses, spread over a geographical area of several square miles, and

they generally include some form'of alarm device or intrusion detector for controlling the operation of the transmitters.

A problem exists with systems of the foregoing type since they may be rendered wholly or partially inoperative by abnormal conditions in the line constituting the loop. In some systems, an opening and/or grounding of the line at any point in the loop can leave all of the stations unprotected, while in others protection is interrupted only for the stations on one side of the opening and/or grounding.

There is, therefore, a need for a new and improved security alarm system which overcomes the foregoing and other problems encountered with the systems heretorfore provided.

SUMMARY AND OBJECTS OF THE INVENTION In the alarm system of the present invention, the first end of the loop is connected to the first terminal of a source of direct current. The second terminal of this source is grounded. Automatic line conditioning means is provided for' monitoring the condition of the line and switching the second end between the two terminals of the power source in accordance with the monitored condition. When the line condition is normal, the second end is connected to the second or grounded terminal of the power source, and upon the occurrence of an opening and/or grounding in the loop the second end is connected to the first terminal. In this manner, the system is maintained in an operable condition despite the occurrence of an opening and/or grounding. Recording pens are provided at each end of the line, and the transmitted signals are recorded by at least one of these pens regardless of the condition of the line. Indicator lights are provided for giving a visual indication of the occurrence of an opening and/or grounding in the loop. Upon correction of the opening and/or grounding, the normal polarity of the applied voltage is automatically restored.

It is in general object of the present invention to provide a new and improved security alarm system of the type utilizing a McCulloh loop.

Another object of the invention is to provide a security alarm system which includes means for automatically reversing the polarity of the voltage applied to one end of the loop upon the occurrence of an opening and/or grounding.

Another object of the invention is to provide an alarm system of the above character which includes means for automatically reversing the polarity of the voltage applied to the loop so that the normal polarity is restored after the opening and/or grounding is corrected.

Another object of the invention is to provide an alarm system of the above character which includes recording pens attached to both ends of the loop for recording alarm signals regardless, of the condition of the. loop.

Additional objects and features of the invention will be apparent from the'following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of one embodiment of a protective alarm system incorporating the present invention.

FIG. 2 is a schematic diagram of the monitoring station portion of the system illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in FIG. 1, the system includes a protective loop 10, a monitoring station 11, and a plurality of protected stations each having a transmitter 12.

The loop 10 comprises an electrically conductive line connected at its ends 10a, 10b to the monitoring station 11 in a manner hereinafter described. The transmitters 12 are connected electrically in series with the line intermediate the ends 10a, 10b. While, for convenience of illustration, the transmitters of only two protected stations are shown in the drawing, the monitoring station 11 can accommodate on the order of 50 such transmitters. These transmitters are located remotely of the monitoring station and can be spread over an area of several square miles.

Each of the transmitters 12 includes means for generating a pulse coded alarm signal which alternately opens and grounds the line constituting loop 10 for brief intervals. The signals from each of the transmitters are coded differently to permit identification of the transmitting station at the monitoring station. Each of the protected stations includes means for detecting an alarm condition and actuating the local transmitter in response thereto. One particularly suitable system for detecting alarm conditions for controlling the transmitters 12 is disclosed in copending application, Ser. No. 50,233 filed June 26, 1970 and assigned to the assignee of the present invention.

The monitoring station 11 includes a length of recording tape 16 wound upon a storage reel 17 and a motor driven capstan 1 8 cooperating with an idler wheel 19 for drawing tape from the reel 17. A pair of recording pens 21, 22 are provided for recording the signals from the transmitters 12 on thetape 16. These pens are connected to the ends 10a, 10b of the loop in a manner hereinafter described.

The monitoring station 11 is illustrated in greater detail in FIG. 2 and can best be described with reference to this figure. This station includes adirect current power supply 26 having a negative output terminal 27 and a positive output terminal 28. The positive output terminal is connected to an earth ground. In the preferred embodiment, the supply 26 is a conventional AC operated supply which delivers a DC output voltage on the order of volts.

The end 10a of the protective loop is connected to the negative terminal of the power supply 26 through a resistor 29, the

operating coil 31a of a relay 31, and a constant current source 30 which is described more fully hereinafter. In FIG. 2, most of the connections to the terminals of the power supply have been omitted for ease of illustration. However, it is to be understood that all terminals marked with a minus sign are connected to the negative terminal 27 and all terminals marked with a plus sign are connected to the positive terminal 28.

Automatic line conditioning means is provided at the second end 10b of the loop 10 for monitoring the condition of the loop and for connecting the end 10b to the terminals of the power supply in accordance with condition of the loop. This means includes an NPN switching transistor 32 and a relay switch 33. The relay switch includes an operating coil 33a, a pair of terminals 33b and 33c, and a contact 33d movable between the terminals 33b and'33c. The contact 33d is connected to the end 10b through a resistor 34 and the operating coil 36a of another relay 36. The emitter of the transistor 32 is connected to the positive terminal 28 of the power supply, and the collector of this transistor is connected to one end of a resistor 37. The other end of this resistor is connected to one end of the relay coil 36a, and the other end of this coil is connected to the negative terminal 27 of the power supply. A control signal is provided for the transistor 32 by means of a resistor 38, zener diode 39 and resistor 41 connected in series between one end of the relay coil 36a and the base of the transistor 32. A pair of diodes 42, 43 are connected in series between the emitter of the transistor 32 and the junction of the zener diode 39 and resistor 41. This same junction is connected to the terminal 33:: of the relay 33. The terminal 33b of this relay is connected to the negative terminal 27 of the power supply through a zener diode 46, a constant current source 49 and a resistor 47. A capacitor 48 is connected across the relay coil 33a for delaying the operation of the relay 33 so that the normal pulses in the alarm signals from the transmitters 12 will not cause the polarity of the voltage applied to the ends 10b of the loop to be reversed.

Means is provided for actuating the capstan 18 and the recording pens 21 and 22 in response to alarm signals from the transmitters 12. This means includes the relay switches 31 and 36 for controlling the pens 21 and 22, respectively. In addition to its operating coil, each of these relay switches includes a pair of terminals designated by the suffixes b and and a movable contact designated by the suffix d".

The means for actuating the pen 21 also includes a pair of transistors 51, 52 connected as a Darlington pair and a solenoid coil 21a for moving the pen 21 in a direction normal to the direction of travel of the tape 16. The relay switch terminal 310 is connected to the positive terminals 28 through a resistor 23, and the contact 31d is connected to the negative terminal 27 through a capacitor 54 shunted by a resistor 56. The remaining terminal 31b of this relay switch is connected to the base of the transistor 51 through a resistor 57. Biasing for this transistor is supplied through a resistor 58 connected between its base and the negative supply terminal 27. The solenoid coil 21a is connected between the positive supply terminal 28 and the junction of the collectors of the transistor 51, 52. The emitter of the transistor 52 is connected to the negative terminal 27. A clamping diode 59 is connected across the solenoid 21a for limiting the inductive voltages across this coil.

In addition to the relay switch 36, the means for actuating the pen 22 includes a Darlington pair comprising transistors 61 and 62 and a solenoid coil 22a for moving the pen 22. The terminal 36c of the relay switch 36 is connected to the positive terminal 28 through a resistor 63, and the contact 36d is connected to the negative terminal 27 through a capacitor 64. The terminal 36b is connected to the base of the transistor 61 through a resistor 66, and a biasing resistor 67 is connected between the base of this transistor and the' negative terminal 27. The solenoid coil 22a is connected between the positive terminal 28 and the junction of the collectors of the transistors 61 and 62. The emitter of the transistor 62 is connected to the negative supply terminal 27, and a clamping diode 68 is connected across the solenoid coil 22a.

The tape drive capstan 18 is driven by a motor 71, and means is provided for energizing this motor upon receipt of an alarm signal and holding it on for a predetermined period thereafter. This means includes a timing capacitor 72 having a variable resistor 73 and a fixed resistor 74 connected in series across it. One end of this capacitor is connected to the positive supply terminal 28, and the other end is connected for receiving charging current from the outputs of the Darlington pairs through diodes 76, 77 and a resistor 78. A transistor 79 is controlled by the voltage on the capacitor 72. This transistor has a resistor 81 connected between its collector and the negative terminal 27 and a resistor 82 connected between its emitter and the positive terminal 28. The base of the transistor 79 is connected to one end of the capacitor 72 through a resistor 83. The collector of the transistor 79 is also connected to the base of another transistor 84. A resistor 86 is connected between the emitter of this transistor in the negative terminal 27, and a capacitor 87 is connected between the collector and base. One terminal of the terminal 71 is connected to the collector of the transistor 84 through a resistor 88, and the other terminal is connected to the positive terminal 28. The zener diode 89 is connected between the two terminals of the motor.

Means is provided for giving a visual indication of grounding and/or opening in the loop 10. This means includes a lamp 91 for indicating an open condition, a lamp 92 for indicating grounding, and a lamp 93 for indicating simultaneous opening and grounding. A transistor 94 is provided for controlling the energization of the lamp 91. The lamp 91 is connected between the collector and emitter of this transistor. The emitter is connected to the negative terminal 27, and the collector is connected to the positive terminal 28 through a resistor 96. The base of this transistor is connected to the end 10a of the loop through the current source 30 and a resistor 97.

A transistor 101 is provided for controlling the energization of the lamp 93. The emitter of this transistor is connected to the negative terminal 27, and the collector is connected to the positive terminal 28 through a resistor 102. The base is connected to the movable contact 36d of the relay switch 36 through a resistor 103. One terminal of the lamp 93 is connected to the collector of the transistor 101, and the other terminal is connected to the anode of a diode 104. The cathode of this diode is connected to the junction of the resistor 96 and the collector of the transistor 94.

A transistor 106 is provided for controlling the energization of the lamp 92. The emitter of this transistor is connected to the negative terminal 27, and the collector is connected to one terminal of the lamp 92. The other terminal of the lamp is connected to the positive terminal 28 through a resistor 107. The base of the transistor 106 is connected to the current source 49 through a resistor 108.

Operation and use of the protective alarm system can now be described. Initially, let it be assumed that the condition of the loop 10 is normal, that is there is no opening or grounding in the loop. in this condition, the transistor 32 is turned on and the relay coil 33a is energized, connecting the contact 33d to the terminal 330. Thus, the end 10b of the loop is connected to the positive terminal 28 through the relay switch 33 and diodes 42, 43. Current flows through the entire loop, and the relay coils 31a and 36a are energized, connecting the contacts 31d and 36d to the terminals 310 and 36c, respectively. Thus, the capacitor 54 is charged through the resistor 53 and relay switch 31, and the capacitor 64 is charged through the resistor 63 and the relay switch 36.

Next let it be assumed that an alarm signal is transmitted by one of the transmitters 12. This signal consists of a plurality of pulses formed by momentary opening and grounding of the loop. Each momentary opening causes momentary de-energization of the relay coils 31a, 360. Thus, each pulse causes the contacts 31d and 36d to be connected momentarily to the terminals 31b and 36b. During each of these momentary connections, The voltage is stored on the capacitors 54 and 64 are applied to the bases of the transistors 51 and 61 in the Darlington pairs. The outputs of these pairs are applied to the motor control circuit, and the first pulse in each signal causes the motor 71 to be turned on. The motor remains on for the time period determined by the values of the capacitors 72 and the resistors 73, 74. In the preferred embodiment, the values of these components are chosen such that the motor remains on for a period of time somewhat greater than the duration of the normal alarm signal. The outputs of the Darlington pairs are also applied to the solenoid coils 21a and 22a so that each pulse causes a deflection of the recording pens 21 and 22 in a direction normal to the direction of travel of the tape 16, thereby recording the pulses on the tape.

During these normal conditions, none of the lamps 91, 92, 93 is energized. The transistor 94 is turned on by the voltage drop across the emitter resistor in the current source 30, thus bypassing the lamp 91. The transistor 101 is turned on by the positive voltage applied to the capacitors 64, reversed biasing the diode 104 and preventing the flow of current in the lamp 93. The transistor 106 is biased to cut off by the negative voltage applied through the resistor 47 and, hence, the lamp 92 is extinguished.

Next let it be assumed that a short develops somewhere in the loop between the line and ground. In this condition, the signals from the stations between the grounding and the end a of the loop will be recorded by the pen 21 as though conditions were normal. At the other end of the loop, however, the transistor 32 is biased to cut off, and the relay coil 33a is de-energized as soon as the capacitor 48 has time to discharge. This causes the contact 33d to be connected to the terminal 33b applying a negative voltage to the end 10b through the resistor 47, zener diode 46, and relay switch 33. Since the positive terminal of the power supply is grounded, current con-- tinues to flow in the portion of the loop between the short and the end 10b. Even though this current is opposite in direction to the current which flows during normal conditions, the pen 22 continues to record the alarm signals from this portion of the line as though conditions were normal.

The occurrence of the grounding does not change the states of the transistors 94 and 101, and therefore the lamps 91 and 93 remain extinguished. The transistor 106, however, is turned on by the voltage drop across the resistor 47, and the grounding condition indicator lamp 92 is energized.

Next let it be assumed that an open circuit occurs somewhere in the loop. Once again the pen 21 functions in the normal manner for signals originating between the opening and the end 10a of the loop. The transistor 32 is again turned off, the coil 33a is de-energized, and the loop end 10b is connected to the negative terminal 27 through the relay switch 33. Thus, the pen 22 records the alarm signals originating between the opening and the end 10b.

In this condition, current does not flow through the coil 31a except during the transmission of pulses. Hence, there is no voltage drop across this coil, and the transistor 94 is turned off. The lamp 91 receives energizing current through the resistor 96 and indicates that the loop is open. At the other end of the loop, the coil 36a is likewise de-energized, and after the capacitor 64 has discharged through the resistor 66 and 67, the transistor 101 is turned off. In this condition, both terminals of the lamp 93 are connected to the positive terminals 28 through the resistors 96 and 102, and the lamp remains extinguished. The base of the transistor 106 is connected through the resistor 108 to a voltage divider between the positive and negative supply terminals..This divider includes the diodes 43 and 42, zener diode 39, resistor 38, zener diode 46, the constant current source 49, and resistor 47. The values of the breakdown voltages of the zener diodes are chosen to be sufficiently large that the voltage drop across the resistor 47 is not sufficient to turn the transistor 106 on. Accordingly, the lamp 92 remains extinguished.

Next let it be assumed that opening and grounding occur simultaneously in the loop 10. This condition would be produced, for example, if the line were to break and one of the broken ends were to contact the earth. Here let it be assumed that the grounding occurs between the opening and the loop end 10a. The pen 21 continues to record the signals originating between the opening and the end 10a, and the pen 22 continues to record the signals originating between the opening and the end 10b. Since current flows through the coil 31a, the transistor 94 is turned on, and the lamp 91 is extinguished. In the absence of an alarm signal, no current flows through the coil 36a, and the transistor 101 is turned off. Thus, one terminal of the lamp 93 is connected to the positive terminal 28 through the resistor 102, and the other terminal is connected to the negative terminal through the diode 104 and the transistor 94. This lamp is thus illuminated to indicate the opening and grounding of the loop. The lamp 92 remains extinguished for the same reason it remains extinguished during the simple open circuit condition.

Upon removal or correction of the opening and/or grounding in the loop 10, the transistor 32 is again turned on, energizing therelay coil 33a, thus automatically connecting the end 10b of the loop back to the positive terminal 28 of the power supply.

The constant current supplied 30 and 49 provide current and short circuit protection for the system. In a typical McCulloch system of the type heretofore provided, current in the loop is maintained at a fixed value by means of at least one potentiometer located in the monitoring station. With this arrangement, the current in the loop varies with voltage fluctuations, necessitating frequent adjustment of the potentiometers. Also, if the line should develop a short or ground fault near the monitoring station, excessive currents can flow and damage the relay coils and other components connected to the line. These problems are eliminated by the constant current sources 30 and 49 in the present invention.

The current source 30 includes a transistor 111 having its collector connected to the bottom end of the relay coil 31a. The emitter of this transistor is connected to the negative terminal 27 through a resistor 112 and to the base of the transistor 94 through the resistor 97. Biasing for the transistor 111 is provided by a network consisting of resistors 113-115 and a thermistor 117. The thermistor provides temperature stabilization for the current source. In the preferred embodiment, it has a negative temperature coefiicient and is mounted on a common heat sink with the transistor 111 to provide close tracking between the collector current and base bias.

The current source 49 includes a transistor 121 having its collector connected to the anode of the zener diode 46 and its emitter connected to the junction of the resistors 47 and 108. Biasing for this transistor is provided by a temperature stabilized network comprising resistors 122-124 and a thermistor 126. This thermistor also has a negative temperature coefficient and is mounted on a common heat sink with the transistor 121.

Since constant current sources are provided at both ends of the loop 10, the current limiting and short circuit protection are available regardless of the condition of the line.

It is apparent from the foregoing that a new and improved protective alarm system has been provided. This system includes automatic line conditioning means for maintaining the system in an operable condition despite openings and groundings in the protective loop, and it includes means for giving a visual indication of theoccurrence of openings and/or groundings. While only one presently preferred embodiment of the invention has been described herein, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

We claim:

1. In a security alarm system, an electrically conductive loop having first and second ends, at least one transmitter connected intermediate said ends for transmitting alarm signals through said loop by alternately opening and grounding the same, first recording means operably connected to the first end of said loop for recording alarm signals received at said first end, second recording means operably connected to the second end of said loop for recording alarm signals received at said second end, a direct current power source having a first terminal connected to said first end and a second terminal connected to ground, said terminals being of opposite electrical polarities, and automatic conditioning means at the second end of said loop for monitoring the condition of said loop and connecting said second end to said second terminal when said condition is normal and connecting said second end to said first terminal in response to opening and/or grounding of said loop 2. A security alarm system as in claim 1 wherein said automatic conditioning means includes at least one electronic switching element.

3. A security alarm system as in claim 1 together with indicator means connected to said automatic conditioning means for giving a visual indication of the condition of said loop.

4. A security alarm system as in claim 1 wherein said automatic conditioning means includes means for reconnecting said second end to said second terminal upon correction of an opening and/or grounding of said loop.

5. A security alarm system as in claim 1 wherein said power source includes means for maintaining the magnitude of the current flowing in said loop at a predetermined constant level.

6. In a monitoring station for use with a protective loop comprising an electrically conductive line and at least one transmitter located remotely of said monitoring station for transmitting pulse coded alarm signals over said line by alternately grounding and opening the same, a direct current power source having a first terminal connected to the first end of said line and a second terminal connected to ground, said first and second terminals being of opposite electrical polarities, and automatic line conditioning means connected to the second end of said line for monitoring the condition of said line and connecting said second end to the second terminal of said power source in the absence of an opening and/or grounding of said line and for connecting said second end to the first terminal of said source in response to an opening and/or grounding of said line.

7. A monitoring station as in claim 6 further including a recording tape driven by a motor adapted to be actuated in response to said pulse coded alarm signal and recording means connected to each end of said line for recording alarm signals received at each end on said tape.

8. A monitoring station as in claim 7 further including timing capacitor means connected to said motor for maintaining said motor in its actuated condition for a predetermined period of time after the receipt of the first pulse in said alarm signal.

9. A monitoring station as in claim 6 wherein said automatic line conditioning means includes an electronic switching device and a relay switch, said electronic switching device having first and second terminals and a control element connected to the second end of said line for controlling the conductivity between said terminals in accordance with the condition of said line, said terminals being connected for controlling the flow of current from said source to the operating coil of said relay switch, said relay switch having a pair of terminals each connected to one of the terminals of said power source and a contact member connected to the second end of said line movable between said pair of terminals for connecting said second end to one of said pair of terminals in accordance with the state of energization of said operating coil.

10. A monitoring station as in claim 9 wherein said electronic switching element is a transistor.

11. A monitoring station as in claim 6 together with constant current regulating means at both ends of said loop for maintaining the current supplied to all portions of the line at a predetermined constant level regardless of the condition of the line.

12. A security alarm system as in claim 1 further including a recording chart and motive means for moving said chart in a first direction, said first and second recording means each including a recording pin for recording the alarm signals on said chart.

13. A security alarm system as in claim 12 further including means for engaging said motive means for a predetermined period of time upon receipt of an alarm signal at either end of said loops. 

1. In a security alarm system, an electrically conductive loop having first and second ends, at least one transmitter connected intermediate said ends for transmitting alarm signals through said loop by alternately opening and grounding the same, first recording means operably connected to the first end of said loop for recording alarm signals received at said first end, second recording means operably connected to the second end of said loop for recording alarm signals received at said second end, a direct current power source having a first terminal connected to said first end and a second terminal connected to ground, said terminals being of opposite electrical polarities, and automatic conditioning means at the second end of said loop for monitoring the condition of said loop and connecting said second end to said second terminal when said condition is normal and connecting said second end to said first terminal in response to opening and/or grounding of said loop
 2. A security alarm system as in claim 1 wherein said automatic conditioning means includes at least one electronic switching element.
 3. A security alarm system as in claim 1 together with indicator means connected to said automatic conditioning means for giving a visual indication of the condition of said loop.
 4. A security alarm system as in claim 1 wherein said automatic conditioning means includes means for reconnecting said second end to said second terminal upon correction of an opening and/or grounding of said loop.
 5. A security alarm system as in claim 1 wherein said power source includes means for maintaining the magnitude of the current flowing in said loop at a predetermined constant level.
 6. In a monitoring station for use with a protective loop comprising an electrically conductive line and at least one transmitter located remotely of said monitoring station for transmitting pulse coded alarm signals over said line by alternately grounding and opening the same, a direct current power source having a first terminal connected to the first end of said line and a second terminal connected to ground, said first and second terminals being of opposite electrical polarities, and automatic line conditioning means connected to the second end of said line for monitoring the condition of said line and connecting said second end to the second terminal of said power source in the absence of an opening and/or grounding of said line and for connecting said second end to the first terminal of said source in response to an opening and/or grounding of said line.
 7. A monitoring station as in claim 6 further including a recording tape driveN by a motor adapted to be actuated in response to said pulse coded alarm signal and recording means connected to each end of said line for recording alarm signals received at each end on said tape.
 8. A monitoring station as in claim 7 further including timing capacitor means connected to said motor for maintaining said motor in its actuated condition for a predetermined period of time after the receipt of the first pulse in said alarm signal.
 9. A monitoring station as in claim 6 wherein said automatic line conditioning means includes an electronic switching device and a relay switch, said electronic switching device having first and second terminals and a control element connected to the second end of said line for controlling the conductivity between said terminals in accordance with the condition of said line, said terminals being connected for controlling the flow of current from said source to the operating coil of said relay switch, said relay switch having a pair of terminals each connected to one of the terminals of said power source and a contact member connected to the second end of said line movable between said pair of terminals for connecting said second end to one of said pair of terminals in accordance with the state of energization of said operating coil.
 10. A monitoring station as in claim 9 wherein said electronic switching element is a transistor.
 11. A monitoring station as in claim 6 together with constant current regulating means at both ends of said loop for maintaining the current supplied to all portions of the line at a predetermined constant level regardless of the condition of the line.
 12. A security alarm system as in claim 1 further including a recording chart and motive means for moving said chart in a first direction, said first and second recording means each including a recording pin for recording the alarm signals on said chart.
 13. A security alarm system as in claim 12 further including means for engaging said motive means for a predetermined period of time upon receipt of an alarm signal at either end of said loops. 